RFR: JDK-8283648: Improve the snippet "file not found" message.

[Jonathan Gibbons]

Please review a (mostly) resource-file change to improve the message when the file for an external snippet cannot be found.

Previously, the code (re)used an existing resource, "File not found". This resource was used in conjunction with files specified on the command line, like a help file or a stylesheet file.   Thus, little extra context was needed.

For external snippets, the underlying problem is less overtly connected to anything specific on the command line, although the problem may be on the command line or related to info on the command line.  Thus, a new resource is created specifically for external snippet files, that hints at issues with (files on) the source path or snippet path.

The main snippet test is updated.

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Commit messages:
 - JDK-8283648: Improve the snippet "file not found" message.

Changes: https://git.openjdk.java.net/jdk/pull/7951/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7951&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283648
  Stats: 9 lines in 3 files changed: 1 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7951.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7951/head:pull/7951

PR: https://git.openjdk.java.net/jdk/pull/7951